For simplifying the production of containers, particularly of aerosol cans including a decoration, some individual treatment steps of the current production methods can be simplified or, in some cases, omitted or replaced. For example, a method is known from WO 95/34474 wherein one can substantially do without imprinting and overpainting of the outer surface of a container. The decoration is imprinted onto a film, and the film is then applied to the outer surface of the container.
By applying the decoration in the form of a film, various advantages are achieved. First, it is no longer required to run all operations necessary for the production of a container immediately one after the other. The printed film can be bought in completely imprinted form and does not need to be printed within the can production plant. One can economize in the cleaning procedure, because the application of a ready, sheet-like layer is less delicate than lacquering, particularly with respect to possible fatty residues. A further economization is obtained by omitting drying procedures. By omitting various parts of an installation, investment costs are also reduced.
Approaches where a label is wound around and fixed to a container have the disadvantage that arranging and fixing cannot be made very precisely. Such a label film does not adhere equally well in all places on the container so that the film can, in some cases be torn off. Residues of adhesive in the overlapping zone and insufficient engagement on the container result in an unattractive appearance.
According to other approaches, closed shrink films are shrunk on the containers. If a hose-shaped intermediate product or sleeve is produced from a printed shrink film, a film ribbon has to be wrapped around in such a manner that the two lateral margins meet each other in an overlapping manner. The overlapping lateral margins are interconnected by way of an applied adhesive. The closed ribbon is pressed flat, and is rolled up with two lateral folding lines. When producing a hose-like film ribbon, there will occur peripheral inaccuracies.
In addition, the adhesive used results in optical deficiencies, and the two foldings, which result from pressing flat and rolling up, remain visible on the container. Since the shrinking capability of a film hose is increased in its longitudinal direction, a hose portion applied to a container would shrink more in the direction of the container's axis than in peripheral direction, which renders a precise engagement of the film hose more difficult. In order to be able to guarantee sufficient shrinking also in peripheral direction, a thick film had to be chosen, which involves higher costs and an undesirable high step in the overlapping zone.
Apart from various can shapes, various methods for producing can bodies are known. In the case of one-part aerosol cans of aluminum, the cylindrical can body is provided by cold sinking. Subsequently, a valve seat is formed at the open end by jolt-necking. U.S. Pat. No. 4,095,544 and EP 0 666 124 A1 describe the production of weldless steel cans. In doing so, the cylindrical can body is produced by stamping, pressing and flow turning a steel sheet coated with tin or with plastic material.
Very current are also cans from steel sheet material, where the shell includes a longitudinal weld seam. The bottom and the upper closing are fastened to the shell by folded seam connections. From documents EP 200 098 A2 and EP 208 564, two part cans and multi-part cans are known, where the parts are interconnected by laser welding. Due to the various shapes and the extremely thin can wall of the individual types of cans, it is not suitable to wind the decoration film directly onto the can body, and to connect it to form a closed film envelope on the can body.
Connecting the film ends by way of an adhesive would be too expensive and would not satisfy both with respect to strength and esthetical aspects. To interconnect the film ends by a sealing connection, the sealed surface had to be pressed against the can body which is not suitable with thin cans due to the small strength. With cans, whose outer surface is narrowed at the lower end and particularly at the upper end or which deviate from a cylindrical shape, the sealing connection could not be attained over the whole height of the can.
From the documents U.S. Pat. No. 4,199,851 and DE 197 16 079, solutions are known where a shrinkable flat plastic material is wound around a winding mandrel, is formed to closed envelopes, are slipped upwards in axial direction onto bottles as an all around label and are shrunk on. The all around labels have to have sufficient stability in order to be able to be moved by an annular part pushing the lower edge upwards up to a label region of the bottles. This is only possible if using plastic material of great thickness.
From EP 397 558 A1, a solution is known, wherein all around labels are clamped by two partially annular pincers portions against an inner part, and are drawn downwards over a bottle. Since the all around label has to be drawn over the solid inner part and over the bottle due to the static friction at the pincers portions, this solution too can only be applied with extremely strong or thick all around labels, whose inner surface, in addition shows good sliding properties.
In accordance with EP 547 754 A2, all around labels are tentered from the inner side and drawn over a bottle by holding rods oriented in parallel and moved in radial direction. Tentering by rods, which press to outside, is only possible with sufficiently robust, and thus thick, films. Thin films would be deformed and/or damaged.
In order to reduce friction on the bottle, air is blown out of the rods. When a desired position relative to the surface has been reached, pincers-shaped grippers, that are moved from outside, have to clamp the all around label so that the rods arranged in the interior can be pulled out without displacing the all around label. The transfer of all around labels onto bottles, known from the prior art, is expensive and not suited for thin films.
Since labels on bottles extend only over a middle zone, the bottles may be held without any problem in an upper or lower region, preferably at a thread at the opening, when applying all around labels. If a film is applied as a decorating layer onto a can, the film has substantially to cover the whole area of the external surface so that holding, as with bottles, is not possible.
In order to arrange a closed film envelope on a can body, the leading edge, according to EP 1 153 837 A1, is held on a winding mandrel. After winding up the film piece, the trailing edge of the piece of decorated film should overlap the leading edge. In the overlapping zone, a narrow sealing surface extending parallel to the longitudinal axis of the can is moved radially inwards and towards the winding mandrel.
In order to enable a sealing connection in the overlapping zone, the film pieces have to be printed and cut in such a manner that a non-printed area exists at the leading edge, and in some cases at the trailing edge. In the non-printed area, the sealing layers formed on both sides of the plastic film should coincide and form a firm connection after sealing.
Now, it has been found that with plastic films after printing, due to their elasticity, the presence in certainty of narrow, non-printed areas at the leading edge, and in some cases at the trailing edge cannot be guaranteed. The reason is that the films as long webs have to be unwound from coils when printing and cutting. The force necessary for unwinding varies from the beginning of the coil to the end. Therefore, the elongating force acting on the film varies too, which leads to the above-mentioned inaccuracies. If the non-printed area were enlarged for compensating the inaccuracy, there would be an undesirable longitudinal strip without any decorating layer on the can body provided with the film.
When the sealing surface, which extends parallel to the longitudinal axis of the can, is moved radially inwards and towards the winding mandrel, it is difficult to ensure equal contact pressure along the whole sealing surface. Since the can body has to be associated to the winding mandrel at one front side for transferring a film envelope, the winding mandrel can only be supported at one front side. Due to the one-sided support, a small tilting excursion of the winding mandrel away from the sealing surface will result when pressing the sealing surface on.
A small sliding movement caused by it between the winding mandrel and the sealing surface as well as the contact pressure that changes in direction of the axis of the winding mandrel can lead to a draft, in some cases to undulating in the overlapping and sealing area and to insufficient sealing. This may affect the esthetic effect of the decorated film even after transfer and shrinking on the can body. Moreover, it has been found that pressure lines or moulds of the lateral marginal lines of the sealing surface may occur in axial direction of the winding mandrel at both sides of the sealing surface, which are even visible on the can body.
With the thin decorated films mentioned in EP 1 153 837 A1 having a thickness of less than 25 μm, preferably between 9 μm and 21 μm, great problems arise when sliding the closed film envelopes from the winding mandrel to the can body. The commercial plastic film Label-Lyte ROSO LR 400 of Mobil Oil Corporation able to be imprinted, as suggested, includes a thin sealing layer on both sides and is available with thicknesses of 20 μm and 50 μm.
When sealing the overlapping area, the sealing layer engaging the winding mandrel is also heated and is pressed to the winding mandrel. In order to avoid that the cylindrical closed film on the winding mandrel is sealed to the winding mandrel, the sealing layer and the outer surface of the winding mandrel have to be provided in such a manner that they do not adhere to each other after a sealing procedure.
Nevertheless, the adhering and sliding properties are somewhat different in the area of the sealing seam as compared with the other film areas. This may lead to problems when moving the film envelopes from the winding mandrel to the can body. If a partial area of the film envelope slides a little bit less well from the winding mandrel to the can body, it may jam on the winding mandrel or on the can body.
Further problems may arise due to electrostatic charges and the electrostatic forces involved which act onto a film. Therefore, the transfer of a cylindrical closed film from a winding mandrel to a can body is problematic, even if the diameter of the winding mandrel is a little bit larger than the diameter of the can body. A significant difference in size is not desirable, because in this case the ability of the film to shrink has to be larger so that there is the risk of undulating when shrinking. For increasing the shrinking ability, a film of an elevated thickness has to be used, which is not desirable.